EOF Exception for ObjectInputStream test

I am trying to test readObject method for reading object from ObjectInputStream

@Test
public void testReadObject() throws Exception {
    ObjectInputStream in = new ObjectInputStream(System.in);
...
} 

I am getting java.io.EOFException for ObjectInputStream in ObjectInputStream in = new ObjectInputStream(System.in);.

Do I mock ObjectInputStream or is it a bad practice? How do I approach in writing a test case to cover readObject scenario as a whole?

My Employee class:

public class Employee {

private Object Teacher;
...
private void readObject(java.io.ObjectInputStream in) throws Exception {
    Teacher = in.readObject();
 }
...
}

Note that I am using JUnit and Mockito.

My method require a little bit more test code, but it gives you an opportunity to write a test without mocking ObjectInputStream:

@Test
void testReadObject() throws Exception {
    String teacherObject = "TeacherObject";
    final ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
    new ObjectOutputStream(outputStream).writeObject(teacherObject);
    final Employee employee = new Employee();

    employee.readObject(
        new ObjectInputStream(
            new ByteArrayInputStream(
                outputStream.toByteArray()
            )
        )
    );

    assertEquals(teacherObject, employee.getTeacher());
}

Note: This test is written under two assumption:

1. Employee class has a method getTeacher(). Cause you need assert somehow that Teacher is correctly read from ObjectInputStream. If you have the other way to do that - modify my example according your need.
2. Employee.readObject() method visibility is public. This is done solely to simplify my example. In your quesion readObject() has a private visibility. You can either modify my example according your need or give me more context that I can modify my example accordingly.

Full source code:

1. Employee.java

package dev.iakunin.stackoverflow.question;

public class Employee {

    private Object teacher;

    public void readObject(java.io.ObjectInputStream in) throws Exception {
        teacher = in.readObject();
    }

    public Object getTeacher() {
        return teacher;
    }
}

2. EmployeeTest.java

package dev.iakunin.stackoverflow.question;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.logging.Level;
import java.util.logging.Logger;
import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;

class EmployeeTest {

    @Test
    void testReadObject() throws Exception {
        String teacherObject = "TeacherObject";
        final ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        new ObjectOutputStream(outputStream).writeObject(teacherObject);
        final Employee employee = new Employee();

        employee.readObject(
            new ObjectInputStream(
                new ByteArrayInputStream(
                    outputStream.toByteArray()
                )
            )
        );

        assertEquals(teacherObject, employee.getTeacher());
    }
}